1. Field of the Invention
The present invention relates to a variable-speed driving system for driving a 3-phase induction motor mounted on an electric car.
2. Description of the Relates Art
FIG. 1 is a circuit diagram of a variable-speed driving system disclosed, for example, in Japanese Patent Laid-Open No. 63-23589. Referring to this Figure, 3-phase-inverters 6 and 7 are series-connected to a DC power supply 1 of, for example, 750 V, through a breaker 2 and a filter reactor 3. A filter capacitor 4 is connected to the input side of the inverter 6 in parallel therewith. Similarly, a filter capacitor 5 is connected to the input side of the inverter 7 in parallel therewith. Primary windings 81 and 82 of a 3-phase induction motor 8 are connected to the outputs of the inverters 6 and 7, respectively. As the breaker 2 is turned on, the filter capacitors 4 and 5 are charged with voltages which are obtained by dividing the DC voltage of the DC power supply at a ratio of 1:1. The 3-phase inverters 6 and 7 convert the divided D.C. voltage into 3-phase A.C. voltages and apply these voltages to the corresponding primary windings 81 and 82 of the 3-phase induction motor 8. A rotating magnetic flux formed by the current in the primary winding 81 and a rotating magnetic flux formed by the current in the primary winding 82 are magnetically combined so as to form a composite rotating magnetic flux. A rotor (not shown) of the 3-phase induction motor 8 is rotated by torque generated by the inter-action between the composite rotating magnetic flux and secondary current which is induced in a secondary circuit (not shown) of the motor.
In FIG. 1, a character I represents the current flowing in the filter reactor 3, E.sub.c1 represents the voltage between both terminals of the filter capacitor 4, E.sub.c2 represents the voltage between both terminals of the filter capacitor 5, I.sub.1 and I.sub.2 represent respective D.C. currents flowing into the 3-phase inverters 6 and 7, P.sub.1 represents the output of the 3-phase inverter 6 and P.sub.2 represents the output of the 3-phase inverter 7. It is assumed here that a condition P.sub.1 &gt;P.sub.2 exists due to unbalance of characteristics between two 3-phase inverters 6 and 7 and the unbalance of impedance between to primary windings 81 and 82. Neglecting the internal losses of the 3-phase inverters 6 and 7, the following two conditions are met in the initial state of the motor. EQU P.sub.1 =E.sub.c1 .times.I.sub.1 &gt;P.sub.2 =E.sub.c2 .times.I.sub.2( 1) EQU P.sub.1 +P.sub.2 =(E.sub.c1 +E.sub.c2).times.1 (2)
If the condition E.sub.c1 =E.sub.c2 is met in the initial state, the following conditions are derived from the formulae (1) and (2). EQU I.sub.1 &gt;I&gt;I.sub.2
Therefore, the voltage E.sub.c2 across the filter capacitor 5 increases while the voltage E.sub.c1 across the filter capacitor 4 decreases. The decrease in the voltage E.sub.c1 and increase in the voltage E.sub.c2 continue till the condition of I.sub.1 =I.sub.2 =I is obtained, and a balance is attained when the following condition is met. EQU P.sub.1 =E.sub.c1 .times.I&gt;E.sub.c2 .times.I=P.sub.2
This causes an unbalance between the D.C. voltages applied to the 3-phase inverters 6 and 7, resulting in an unbalance between the outputs of the 3-phase inverters 6 and 7.
Thus, in the conventional variable-speed driving system, unbalance between the outputs of the 3-phase inverters 6 and 7 is often caused by various reasons such as difference in the characteristics of components such as switching devices (not shown) in the parallel 3-phase inverters 6, 7 and unbalance of impedance between two primary windings of the 3-phase induction motor 8. This unbalance of the outputs often causes accidents such as breakdown of the components of the 3-phase inverters.